The traditional way to pre-load or adjust a simple wheel bearing on a wagon or other piece of farm equipment is to spin the wheel hub and then tighten the nut against the pre-lubricated bearing until the hub begins to drag and slows. Stop the wheel from turning, reverse the nut until it's loose, then re-tighten the nut "hand-tight" before installing a cotter key in the castle nut.
I always thought that sounded rather imprecise, but when I had opportunity to talk with Carl Bush of Wilwood Engineering, the company that makes brakes systems and wheel hubs for NASCAR teams, he said the traditional way of tightening wheel bearings is more than adequate. "You can use a torque wrench and make it more complicated, but for simple wheel bearings, the old way is just fine," he says.
There are all sort of specifications available on the internet for four-wheel-drive, front-wheel-drive and particular designs of wheel hubs. But plain ol' wheel bearings on mid-'80s Monte Carlos, Camaros and Chevelles have recommendations to be tightened to "...12 lb./ft. of torque before loosening nut, then finger-tightening the nut before installing cotter key." Those are similar to the wheel bearings used on generic farm equipment, so...
Twelve lb./ft. of torque turns out to be just enough torque to create drag when you spin the wheel hub. So if you tighten wheel bearings the way Grandpa taught you, you'll be pretty darned close to manufacturer's recommendations for preloading simple wheel bearings. Sometimes the simple way is simply sufficient.